Preheat the oven to 220ºC, fan 200ºC, gas 7. Pour ½ teaspoon oil into each hole of a 12-hole non-stick mini muffin tin and place in the oven to heat through.
Meanwhile, sift the flour and a pinch of salt into a medium bowl and make a well in the centre. Beat in the eggs until smooth, then gradually add the milk, beating until the mixture has no lumps.
Remove the mini muffin tin from the oven and carefully add ½ tablespoon of the batter into the compartments. Cook in the oven for 15 minutes, until the puddings have risen and are golden in colour. Leave to cool on a wire rack.
Fill each mini Yorkshire pudding with a couple of pieces beef and a little horseradish. Season with black pepper and garnish each with a small sprig of watercress.